At "Open Book" there is a vigorous discussion of Elizabeth Anscombe's "Mr. Truman's Degree" going on. In particular, the dispute is about whether Truman's decision to drop the bomb was justifiable or not.
As one might expect, the basic argument that it was justifiable appears to be that if the bomb had not been dropped more people would have died than did. The trouble with this argument is that it doesn't actually seem to be very relevant: to justify an action in this way you already have to have determined that the action is not itself immoral, which Anscombe denies. Anscombe's point is that deciding to kill innocents is in itself an immoral decision, whatever may follow from it. The fact, if it is a fact, that the bombs saved lives in the long run doesn't affect this question one way or another. It does show, assuming that this was, in fact, the reason for Truman's decision, that the decision was not wholly depraved, but was good as to intended consequence. Intended consequence is indeed part of the evaluation of an action, although it is not the only, or even the most important factor. It is certainly enough to mitigate the wrongness of an action. This is not a high aim to shoot for, however; mitigated wrong is still wrong. Beyond this the saving of lives tells us nothing but that immoral actions can have good effects, which everyone already knows.
And it needs to be noted that there is nothing about this conclusion that makes it "cheap armchair generalship," to use the words of one commenter. Good generalship is about leading men to win wars, in which what counts as winning is determined by legitimate strategic goals. But the legitimacy of the strategic goals is not itself a matter of generalship, but, in a democratic nation, a matter of the spirit of the people and of ethics. It may well be that, once this alternative was refused, every other alternative was harder; but in a war all alternatives are always difficult, and not all alternatives are acceptable even if they make victory easier. Everyone has a line they would draw, so it's not adequate simply to point to lives saved and say that this is an adequate reason; one must show why the action itself is not on the wrong side of the line, when that line is rationally drawn.
And, as Anscombe notes, it is clearly on the wrong side of the line. This is not a condemnation of Truman. It's not my place, and unless you happen to be the Judge at the end of ages, I don't think it's anyone else's place, to weigh out precisely what guilt Truman bears in his decision. That requires information about Truman's reasoning and intentions and burdens to which we do not have full access. But this does not change the fact that what was done was wrong. It was a wrong committed against large numbers of people at Hiroshima; it was a wrong committed against large numbers of people at Nagasaki. In addition, it was a betrayal of the ideals of the United States which, last I heard, was founded on the principle that all human beings are endowed by their Creator with at least a basic right to life, regardless of their citizenship.